[Ifeffit] N-idp

[Matt Newville]

Hi Shan,

Nidp does not (to first order, anyway) depend on the step size used
the data collection.  The step size in k does dictate the highest
frequency (R) that can be measured.  This could, in principle, change
the effective R range that could be used in the Nidp calculation, but
it should never have a serious impact on it.

Usually (at least in my experience), step sizes in k are around
0.05Angstroms-1  when collecting EXAFS data, which would give Rmax  =
pi / (2 * step_k) = 31.4 Angstroms.    Of course, that is
significantly larger R than we can hope to measure, but it does allow
us to do "crude signal processing" steps like linear interpolation,
re-binning of data as we shift E0 and so on without ruining the data
below, say 15.7 Angstroms.

So, for EXAFS it's probably dangerous to have a step size larger than
0.2Angstroms^-1, and not a  great help to have step sizes smaller than
0.05Angstroms.     For what it's worth, IFEFFIT/Artemis more or less
insist on interpolating chi(k) onto a k-grid of 0.05Ang^-1.

All that being said, I have not carefully tested how large the step
size has to be to really impact the EXAFS below, say, 5Angstroms^-1.

Cheers,

--Matt